A number of demands, which are difficult to combine, are raised on materials for the manufacturing of plastic forming tools. Among these demands, in the first place the following ones should be mentioned:
A high and even hardness and accompanying strength, regardless the physical dimensions of the product, a feature which is particular important because the mould cavity usually has to be engraved out of the core of the piece of steel. PA1 A very high hardenability, i.e. through hardened ability also in very large dimensions. PA1 Very homogenous features throughout the whole piece of steel. PA1 A high toughness at room temperature but also at lower temperatures, since plastic moulds often are provided with cooling channels in order to cool the tool between the shots in order to increase the production capacity. PA1 A good mechanical machinability and also a good electrical discharge machinability (EDM). PA1 A good weldability PA1 A good surface hardenability, including a good case-hardenability and surface nitridability, such as ability to be surface hardened through ion nitriding.
It is also desired to simplify the heat treatment and, if possible, to completely eliminate hardening and tempering which substantially influence the total production costs and which also can cause cracks or other defects in the material.
The material which so far has been regarded to be the material which has satisfied the above requirements best is the material which since decades is known under its tradename IMPAX.RTM. SUPREME and which has the following nominal composition: 0.37C, 0.3 Si, 1.4 Mn, 2.0 Cr, 1.0 Ni, 0.2 Mo, 0.008 S, balance iron and unavoidable impurities and accessory elements. This material is tough hardened to 290-330 HB in its delivery condition. Successively, however, demands have been established upon plastic moulding tools having better performance than what IMPAX.RTM. SUPREME can offer. Above all a better hardenability, a higher and more even hardness independent on dimensions, a better weldabilty and electrical discharge machinabilty, and a better toughness are desired.
It has also been suggested in the past that a steel having the nominal composition 0.05 C, 0.3 Si, 2.0 Mn, 3.0 Cr, 1.5 Ni, 0.3 Mo, balance iron and unavoidable impurities, intended i.a. for tubes and for chains, particularly for anchor chains, should be useful also for plastic moulding tools. This steel, however, has not been able to satisfy the above specification of features and has, as far as is known to the applicant, never been used as a plastic moulding steel.